Eddy current testing is a commonly used method of nondestructive testing of metallic objects. In one type of system widely used in such testing, the change in impedance of an active electrical coil as it is passed in close proximity to the test object is used to detect flaws. Any variations in the shape or other characteristics of the test object, not just flaws, produce changes in the eddy current signal. For instance, in testing the tubes in a steam generator of a nuclear steam supply system the massive steel tube sheet in which the ends of the U-shaped tubes are mounted, the metal support plates and antivibration bars which support the intermediate portions of the tubes and even deposits which form on these components, produce variations in the eddy current signals. In addition, wobble of the probe as it passes through the tube generates responses which act as noise in the data. Furthermore, there are various geometries of discontinuties such as cracks, pits, dents and thinning of the tube wall thickness which have different effects on the eddy current signal.
The difficulty with eddy current testing is that it does not produce a clear, easily recognizable representative of the characteristics of a test object, such as for instance, an X-ray picture would provide. In addition, a flaw may occur adjacent to a known feature such as a support plate in the case of the steam generator, and thus its response could be masked by the larger signal. As a result, eddy current analysis of steam generator tubes is performed by a small cadre of vigorously trained, extremely skilled analysts who over a period of time have collected a large-body of "experience based" knowledge.
In performing an eddy current test on steam generator tubes a probe is advanced through the tube and the signals so generated are recorded for later analysis. Tests are made at several frequencies, and in some cases signals at various frequencies are "mixed" in a manner which eliminates selected signals, such as those produced by the support plates, so that signals related to flaws at those locations may be extracted from the data. Most of the data are taken with a differential coil pair which produces signals of the opposite sense as each coil successively passes the same tube feature, although an "absolute" coil which is a single coil paired with a reference coil is also used.
For analyzing the data, the human analyst selects whatever frequency he desires for presentation on a cathode ray tube display. The display includes a strip chart presenting the real and imaginary (quadrature) components of the recorded signal for the entire length of the tube under examination. At the analyst's request; any portion of the strip chart can be displayed simultaneously in the form of a lissajous pattern. A lissajous pattern is a well known display in which the real and imaginary components for successive points in the data are plotted in an X-Y plane. Such a plot forms lobes radiating from an origin with the angular position of the lobe representing the phase angle. The human analysts have developed a highly refined ability to observe the shape and phase angle of the lissajous pattern and determine therefrom with a good deal of confidence the characteristics of the tube represented by that pattern. It should be appreciated that for the most part the lissajous patterns are not perfect geometric figures which adds to the difficulty of interpretation. In order to classify a characteristic, the human analyst may switch to other channels to observe the corresponding portion of the signal. In performing an analysis, the human analyst is capable of recognizing, through experience, the pertinent characteristics of the lissajous figures which indicate critical features while rejecting extraneous indications.
The routine nondestructive eddy current inspection of steam generator tubes has become increasingly important and complex as the need to extract more information from the inspection results has increased. During the past decade several factors have contributed to the transformation of a simple maintenance requirement into one of the most critical tasks of the plant outage. Increasingly the NRC, as well as the plant owner/operator need maximum information on the condition of the tubes in the steam generator. The new breed of digital eddy current instruments and multiple sensor probes can now provide a massive volume of inspection data. The tooling to place the probes in the tubes to collect this data has also become very efficient. Thus the number of tubes interrogated, the number of test variables used, and the number of tubing degradation mechanisms have all increased significantly, but the speed of formulating a meaningful interpretation from this information has lagged behind. The already demanding working conditions of human analysts are exacerbated by the fact that the eddy current inspection is usually on the critical path of the utility's outage schedule.
In an effort to provide more information for plant life extension, the number of tubes inspected at every outage has gradually increased. Recently it has not been unusual to find 100% inspections of the 3000 to 5000 tubes in a typical steam generator while a statistical 5%-10% inspection was considered satisfactory in the past. The data is used not only to establish whether the degradation of the tube or loss of the tube wall exceeds 40%, a level requiring remedial action such as sleeving or plugging, but to monitor (and predict) the various mechanisms of degradation. This information is used to take steps to arrest the degradation mechanism. All of these requirements have placed a large burden on the resources of a single data analyst. As a result, procedures have been established in which a secondary and sometimes even a tertiary review of the same data by independent data analysts has been used in order to increase the level of confidence and divide the process into manageable tasks.